TOKYO — One must accept a risk of radiation exposure when flying in and out of Narita International Airport, the busiest airport in Japan, just east of Tokyo, but perhaps not for the reason you are thinking.

Fukushima Daiichi, the tsunami-damaged nuclear reactor site about 150 miles (241 kilometers) to the north, as the foolish crow flies, continues to leak trace amounts of radiation. Radioactive iodine-131 made it into the water supply here last month. But most, as physics would have it, has since decayed into stable xenon.

So, few in this Tokyo region have been exposed to radiation levels as high as someone just hopping off a plane. The international flyer receives a dose of about 0.10 millisievert, or the amount of ionizing radiation in two dental X-rays, from the sun's radioactive cosmic rays. That means that folks who left Tokyo because of the threat at Fukushima likely received more radiation on the airplane flight than they would have if they had stayed at home.